Embossing is well known in the art. Embossing is a common technique used to join two plies of paper together in order to form a multi-ply laminate. The resulting laminate has properties, such as caliper, flexibility, and absorbency, not attainable from a single ply having twice the basis weight of either constituent ply.
The prior art teaches embossing two plies of paper together. Embossing is accomplished by one of several known embossing processes, such as knob-to-knob embossing or dual ply lamination. The foregoing processes are illustrated by commonly assigned U.S. Pat. Nos. 3,414,459 issued Dec. 3, 1968 to Wells and 5,294,475 issued Mar. 15, 1994 to McNeil, the disclosures of which patents are incorporated herein by reference. Yet another embossing process for joining two plies together is nested embossing, as is well known in the art.
With each of the foregoing embossing processes, embossments are deflected out of the plane of the paper. Such deflection may desirably increase the caliper of that ply, and hence the laminate. Conventional embossing may increase caliper 25 to 135 percent as the emboss pressures deform the fibers out of the plane of the paper.
By embossing out of the plane of the paper it is meant that the embossments extend outwardly from the original thickness of the unembossed paper. Thus, embossments which are deformed out of the plane of the paper extend outwardly from the surface of the paper thereby increasing its caliper. The aesthetic clarity of the embossed pattern is directly proportional to the magnitude of the out-of-plane deformation of these embossments.
There is an associated loss in tensile strength caused by the out-of-plane embossments. A common through air dried substrate, such as that found in CHARMIN bath tissue sold by The Procter & Gamble Company of Cincinnati, Ohio, has suffered a 20 to 40 percent tensile loss during conventional embossing processes. Additionally, prior art embossing often degrades softness. The softness degradation is believed to be due to the tactile sensation caused by the out of plane embossments.
Typical prior art embossing processes rely upon a conventional rubber anvil roll and a steel pattern roll to form the aesthetic pattern. The aesthetic pattern results from the deformation of the fibers out of the plane of the paper when the plies are embossed against the deformable anvil roll.
One prior art attempt to emboss an aesthetic pattern onto paper is illustrated by U.S. Pat. No. 5,436,057 issued Jul. 25, 1995 to Schulz. As illustrated by FIGS. 13-14 of Schulz '057, this attempt requires embossing the paper out of its plane to form the embossments.
A similar attempt in the art is illustrated by European Patent Application 0 668 152 A1 published Aug. 23, 1995 in the names of Kamps et al. Kamps et al. also suffers from the drawback, illustrated by FIG. 10, that the sheets are embossed out of the plane of the paper. Neither Schulz '057 nor Kamps et al. suggests embossing an aesthetic pattern within the plane of the paper.
Other attempts in the art have utilized relatively high embossing pressures. However, such attempts are limited to joining multiple plies of paper together. For example, U.S. Pat. No. 3,377,224 issued Apr. 9, 1968 to Gresham et al. teaches embossing two plies of differentially creped paper together without adhesive. The process requires 1/32 inch square bosses.
A similar attempt is found in U.S. Pat. No. 3,323,983 issued Jun. 6, 1967 to Palmer et al. Palmer et al. teaches an embossing process which fixes together plies of thin creped paper. Neither Gresham nor Palmer et al. suggests embossing a single ply of paper. Instead, each teaching limits the embossing process to joining together two or more plies of paper.
Commonly assigned European Patent Application WO 95/27429 filed Apr. 12, 1995 in the names of Reinheimer et al. teaches a cellulose cloth comprising at least two layers. The layers are joined with an embossed pattern of individual spot shaped impressions which deform and mutually connect the tissues of the cloth. The impressions are formed by embossed spots which originate from the outer layers of tissue and curve concavely inwardly.
In contrast, embossing according to the present invention utilizes only a single ply of paper. The aesthetic pattern resulting from embossing the single ply lies within the plane of the paper.
Furthermore, embossing according to the present invention reduces the associated loss of tensile strength. The tensile strength loss associated with embossing according to the present invention is typically less than 10 percent, and in some cases less than 5 percent.
Furthermore, the present invention decouples pattern clarity and the magnitude of the out-of-plane deformation of the embossments. In the present invention, pattern clarity is not determined by the depth of the embossments. Instead pattern clarity is determined by the reflective nature of the embossments. Particularly, the embossments are often glassined and are more reflective than the unembossed regions of the paper.
Embossing according to the present invention increases the modulus of the paper. The modulus, in grams per centimeter, is the slope of the stress/strain curve of the paper as it is loaded in a tensile testing machine at a constraint elongation rate of one inch per minute, using a two inch gage length and a four inch sample width. The slope is measured at a load of 15 grams per centimeter of sample width.
Accordingly, it is an object of the present invention to provide an embossed paper which does not have out-of-plane embossments. It is also an object of the present invention to provide an embossed paper which does not suffer an undue loss of tensile strength as a result of the embossing process.
It is further an object of the invention to provide a single ply of paper having a visually distinctive embossed pattern. It is finally an object of the invention to decouple the clarity of the emboss pattern from the depth of the embossment.